Electroniciliquid measuring system

ABSTRACT

A FUEL GAUGING SYSTEM OF THE CAPACITANCE ELECTRODE TYPE WHICH ENABLES THE GAUGING OF LIQUID IN A CONTAINER REGARDLESS OF THE POSITION OF THE CONTAINER WITH RESPECT TO A GRAVITATIONAL FIELD. EACH PAIR OF OPPOSING WALLS OF THE CONTAINER IS PROVIDED WITH A PLURALITY OF ELONGATED CAPACITOR PLATES, EACH OF THE PLURALITY OF PLATES BEING ONE OF A PAIR, THE OTHER OF THE PAIR BEING LOCATED IN THE OPPOSITE WALL OF THE OPPOSING WALL PAIRS. EACH PAIR OF PLATES IS CONNECTED TOGETHER AND INTO A MEASURING CIRCUIT SO THAT A READING OF MASS OF FUEL MAY BE OBTAINED AND SO THAT FRINGING EFFECTS ARE REDUCED.

United States Patent O Int. Cl. G01f 23/26 U.S. Cl. 73-304 5 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by. reissue.

ABSTRACT F THE DISCLOSURE A fuel gauging system of' the capacitance electrode type which enables the gauging of liquid in a container regardless of the position of the container with respect to a gravitational field. Each pair of opposing walls of the container is provided with a plurality of elongated capacitor plates, each of the plurality of plates being one of a pair, the other of the pair being located in the 0pposite wall of the opposing wall pairs. Each pair of plates is connected together and into a measuring circuit so that a reading of mass of fuel may be obtained and so that fringing efects are reduced.

The present invention relates to fuel gauging systems and, more particularly, to a capacitance gauging system responding to cryogenic iiuids at all levels and all attitudes.

Accordingly, it is the object of this invention to provide capacitive gauging which is accurate, safe and simple in instrumentation and which is reliable in operation throughout the full region of flight in the earths gravitational ield and in zero gravitational conditions.

It is another object of this invention to provide a capacitive gauging system consisting entirely of surface measurements in which capacitive sensors extend along the inner walls of a fuel tank.

It is another object of this invention to provide a capacitive gauging system which responds to the liquid in the tank rather than to the volume of gas, thus leading to greatly improved accuracies at low levels where accuracy is most often needed.

It is yet another object of this invention to provide a capacitance gauging system to achieve mass readings of the fuel being measured.

It is still yet another object to provide a capacitance gauging system for a fuel tank in which the effects of fringing are substantially reduced.

According to one embodiment utilizing the principles of the present invention, there is provided a fuel tank of substantially square configuration having a plurality of capacitive strip sensors arranged in parallel relation on the inside surface of the tank. The capacitive sensors in each wall of a parallel pair of walls` are connected together and the capacitance across these walls is measured, meanwhile each of the strip sensors on one of the remaining walls of the tank is connected to its corresponding strip sensor on the other of the remaining walls of the tank. With this arrangement a linear electric eld is established at the end surfaces of the tank and the errors which normally exist due to fringing are substantially reduced. It can be shown that if the dielectric material between the walls of the tank is less than 1.5, which is the case for most fuels, a surface capacitive measurement is then capable of i1.5% accuracy.

Re. 27,246 Reissued Dec. 14, 1971 ICC Other objects and advantages will become apparent upon a further reading of the following detailed description taken in conjunction with the drawings, in which:

FIG. 1 is a perspective View in cut-away of a fuel tank utilizing the principles of this invention.

FIG. 2 is a cross-sectional View of the fuel tank according to this invention showing the fuel in one position.

FIG. 3 is a cross-sectional view of the fuel tank according to this invention showing the fuel in another position.

FIG. 4 is a graphical representation illustrating the maximum value for the liquid volume in the tank under maximum adverse conditions.

FIG. 5 is a graphical representation illustrating the maximum error found for maximum adverse conditions.

FIG. 6 is a schematic diagram illustrating the circuit used in this invention.

Referring now to FIG. l there is shown a fuel tank 2 of square coniiguration. The tank 2 has an outside layer of conductive metal 4 and an inside layer of foam rubber 6. A series of parallel capacitor electrode strips A, A', B, B are secured to the foam rubber in the manner shown. FIGURE 6 shows how the strips are connected for a capacitance reading across strips B, B'. This is accomplished by connecting all the B strips 0 9 together and all the B strips 0-9 together. The A, A strips are then connected as will be explained later. It may be shown that for a given quantity of liquid X the maximum capacitance will result when the liquid is located in a vertical slab as shown in FIG. 3. It may also be shown that the minimal capacitance will result when the liquid is oriented as a horizontal slab as shown in FIG. 2. All of the distributions will fall at or within these two limits. Since the distribution of propellants or liquid may be such as to produce any reading between these two limits, their difference must be considered to be a region of ambiguity and,` therefore, of error.

The capacitance between strips B and B may be shown to have the following relationship.

CBBf=capacitance between strips B and B with the liquid oriented as shown in FIG. 3

CBB,H=capacitance between strips B and B' with the liquid oriented as shown in FIG. 2

K=dielectric constant of the liquid X=volume filled with liquid (0X1) Ca=capacitance of strip in a vacuum Now the error (E) will be:

E=CBB'v-CBB'H KC, (1 K) -I-K After algebraic manipulation this equation is simplified to:

E:C,X(1X) (K-1)2 (l -K) +K However, the full scale change in capacitation for X=1.0 is

GAK-1) 0, therefore, is defined as the percent of full scale error, and so:

After further algebraic manipulation and simplification it then appears that the maximum (Xm) occurs for:

FK-JK As a simplifying approximation, the binominal expansion of the form is used to show that X-0.5 as shown in FIG. 4.

Since we are only concerned with values of K less than 1.5 (the values for most fuels) or X 0.5, the approximation is valid for values of Kl less than 0.5. The graphical solution of the equation for Xm is shown in FIG. 4.

Having established the area about X=0.5 as being the worst case, the error is now evaluated 1O0(X)(1N)(K-1)N 25(K-1) 50K-1 K(1-K)+K -0.5(K+1 K+1 Since this error band is unidirectional, a factor of 2 improvement may be assumed by having a bidirectional, thus K 1 PD K 2 where: K=dielectric constant D=density P=specilic polarization For cryogenics the C-M law and experimental data show a near linear relationship between K and D which is more specically described in an application filed by Irving H. Cohn on Mar. 12, 1965, Ser. No. 439,253, having the same assignee as this application. Thus, a capacitance gauging system requires little or no compensating elements in achieving a propellant mass reading. Also, the surface capacitance system utilizing the principles of this invention respond to the liquid in the tank rather than to the volume of gas. This leads to greatly improved accuracies at low levels.

In FIG. 6 a regulated AC source supplies the electrodes A, A current and the electrodes B, B current as shown. [n order to reduce fringing effects the A strips 0-9 are connected to their corresponding A strips 0-9, as sho'wn in FIG. 6. A linear iield is therefore established at the end surfaces of the tank and the errors which formerly existed due to fringing are substantially reduced. At this point the capacitance across B, B' is measured and the resultant signal is Ifed through an appropriate amplilier A and compared with a reference capacitance value Ca. A

suitable volume-mass, VM indicator is then energized by the resultant signal from the amplifier circuit.

Although only one embodiment of the invention has been depicted and described, it will be apparent that this embodiment is illustrative in nature and that a number of modifications in the apparatus and variations in its end use may be effected without departing from the spirit or scope of the invention as delined in the appended claims.

That `which is claimed is:

1. In a capacitance gauging system for indicating the mass of fluid in a square shaped container comprising a plurality of parallel spaced apart capacitance strip electrodes extending along the inside surface of the walls of said container, means supplying an alternating current to said electrodes, each of said electrode strips being electrically connected together on each Wall in one pair of opposing walls, each electrode strip in one of the remaining Walls of said container being electrically interconnected with its corresponding electrode strip on the other of said remaining walls in said container and indicating means connected to said alternating current source for indicating the capacitance across said pair of opposing Walls of said container.

2. In a capacitance gauging system for indicating the mass of lluid in a square shaped container comprising capacitance electrode means extending along the inside surface of a pair of opposing walls of said container, a plurality of parallel spaced apart electrode means extending along the inside surface of the remaining Iwalls of said container, means supplying an alternating current to said electrode means, each of said electrode means on one of said remaining walls being electrically interconnected to its corresponding electrode means on the other of said remaining walls and indicating means connected to said alternating current source for indicating the capacitance across said pair of opposing walls of said container.

3. In a capacitance gauging system for indicating the mass of fluid in a rectangular shaped container comprising a capacitance electrode means extending along the inside surface of a pair of Iopposing walls of said container, means supplying an alternating current to said electrode means and indicating means connected to said alternating current source for indicating the capacitance across said opposing Walls of said container.

4. In a capacitance gauging system for indicating the mass of fluid in a symmetrical shaped container comprising, a capacitance electrode means extending along the inside surface of a pair of opposing walls of said container', means supplying an alternating current to said electrode means and indicating means connected to said alternating current source for indicating the capacitance across said opposing walls of said container.

5. In a capacitancey gauging system for indicating the mass of fluid in a container comprising, a capacitance electrode means extending along opposing inside Surfaces of said container, means supplying an alternating current to said electrode means and indicating means connected to said alternating current source for indicating the capacitance across said opposing walls of said container.

References Cited The following references, cited by the Examiner, are of record in the patented file of this patent or the original patent.

UNITED STATES PATENTS 2,357,023 8/1944 Reid et al. 73-304 2,911,576 11/1959 De Giers 73--304 X 3,180,146 4/1965 Hossack 73-304 X 795,189 5/1958 Great Britain 73-304 S. CLEMENT SWISHER, Primary Examiner 

